Transcript Factors affecting the depth of field for SEM

factors affecting the
depth of field for SEM
Lei Zhang
physics and Astronomy
Introduction
 definition of DOF(depth of field)
 the distance between the nearest and farthest
objects in a scene that appear acceptably sharp
in one image ---(SEM large DOF)
Introduction
 In a conventional equipped SEM, DOF is
mainly changed by these two
independent variables:
1.the final aperture size (radius R)
2.the working distance (WD mm)
controlled by software
1.the final
aperture size
(radius R)
2.the working
distance (WD
mm)
Introduction
 difference between DOF and DOF'
 depth of field: refers to space defined on
the specimen
 depth of focus:refers to space in the
region of an image plane (TEM)
 But SEM use difference way to produce
image from TEM, only need consider DOF
Determination of DOF
 DOF is not affected by the nature of the
sample
 But the requirements for DOF are determined
by the roughness of the specimen in the field
of view and the degree of tilt on the
specimen stage
Determination of DOF
 M (magnification)
 d (the effective signal emission diameter)
 'r' (the resolution capabilities of the human
eye when viewing a VDU or a micrograph at
30cm: r = 0.3mm)
 a (the semi-angle of the electron probe)
 semi-angle of the probe is dependent on:
 R (the radius of the final aperture in a normal
SEM)
 WD (the working distance in mm)
Maths Determination of DOF
 The equation relating these variables is:
D = [(r x 10^3) - d/10^3]/a
 Where,
 a = R/(WD x 10^3)
qualitative effects
 the depth of field is higher when
emission disc is smaller
 the depth of field is higher when
final aperture is smaller
 the depth of field is higher when
working distance is longer
 the depth of field is higher when
SEM is at lower magnifications
the
the
the
the
example
 the effective emission disc is 50 nm in diameter and
the resolution capability of the eye is 0.3 mm
100 μm
200 μm
aperture
Magnifica aperture
tion
(a = 0.005 (a = 0.01
rad)
rad)
30
1.9 μm
995 μm
3000
30000
10 μm
1 μm
5 μm
0.5 μm
300 μm
aperture
(a = 0.015
rad)
663 μm
3 μm
0.3 μm
Table 1. Depth of Field at 10 mm working distance
example
Magnificati
on
30
3000
30000
100 μm
aperture
(a = 0.002
rad)
4.9 mm
25 μm
2.5 μm
200 μm
aperture
(a = 0.004
rad)
2.5 mm
12.5 μm
1.2 μm
300 μm
aperture
(a = 0.006
rad)
1.6 mm
8.3 μm
0.8 μm
Table 2. Depth of Field at 25 mm working distance
Enhancing the Depth of Field
 increasing the working distance
 decreasing the size of the final
aperture
 But largest WD and smallest R are not
best choice! (side effects)
side effects
 increasing the working distance
produces:
• increased depth of field
• lower attainable limits for low
magnification
• some loss of resolution
• a possible decrease of signal strength
• astigmatism will worsen at long WD
• some serious distortions in the image
side effects
 decreasing the size of the final
aperture produces:
 an increased depth of field
 a decrease in probe current
 a possible improvement in the probe
resolution
 a change in astigmatism (needs to be
corrected again
Conclusion
 In most cases, a combination of changes
in working distance and aperture size
will be enough to provide the necessary
range requirements for adjustments to
the depth of field.
 not perfect, but good enough for your
sample